plots: built-in plot functions for HLSM object

Description

plotLikelihood( ) plots the likelihood, and plotDiagnostic( ) plots diagnostic-plot of posterior draws of the parameters from MCMC sample. plotHLSM.random.fit( ) and plotHLSM.fixed.fit( ) are functions to plot mean-results from fitted models, and plotHLSM.LS( ) is for plotting the mean latent position estimates.